Intrusion alarms are devices which generate an alarm signal when an unauthorized entry into a protected structure is detected. One common method for gaining access into a protected structure is to break the glass in a window. In response to this mode of entry, glass break detectors have been developed which generate an alarm signal when the sound of breaking glass is detected.
When glass mounted in the wall of a room is broken by impact, the acoustic signal produced is a function of several variables. These variables include the type of glass, its size, the mounting method, and the acoustic properties of the room. When glass breakage due to a forced entry occurs, the acoustic signal which is generated contains both low frequency and high frequency components. The frequency spectrum of the signal is very wide, ranging from below 3 Hz to well over 20 kHz.
The low-frequency components of the signal are caused by the initial displacement of the glass as it rebounds from the blow which is intended to break the glass. If the mounting frame and wall are flexible, they may contribute to the low-frequency components as well. The high-frequency components of the acoustic signal are generated by the initial impact, the actual fracturing of the glass, and by collisions of glass fragments with each other and with barriers in the room.
Glass break detectors which rely on detecting the acoustic signal generated by breaking glass operate by selectively detecting one or more of the frequency components associated with the event. Some glass break detectors are sensitive to only a narrow band of frequencies in the high end of the spectrum, while others detect high and low frequency components of the signal. Glass break detectors such as model FG-730 manufactured by C & K Systems, Inc. of Folsom, Calif., require two frequency components to have a defined duration and arrive nearly simultaneously before the acoustic signal is identified as a glass break. Sensing multiple rather than single frequency components of a glass-break event reduces the system's probability of generating a false alarm.
However, it has been discovered that intrusion alarms which depend upon the sensing of both low and high frequencies are susceptible to false alarms in the presence of a large high frequency acoustic signal. The electret microphone used in many glass break sensors to detect both the low and high frequency components of the signal is believed to be the source of the problem. This type of microphone is used because at this time it is the only commercially available one which can effectively cover the desired frequency range of 3 Hz to 20 kHz. In most systems, the combined acoustic signal is detected by the microphone and then subjected to filtering in order to separate out the various components. After filtering, the signals are processed, with the output of the signal processing circuits being sent to a logic and timing circuit to determine if an intrusion alarm signal is warranted.
In such single microphone systems it has been found that the microphone's field-effect-transistor (FET) saturates in the presence of a large high frequency acoustic signal. Such a signal causes the FET to operate in a non-linear fashion and to produce low frequency electrical signals in addition to the high frequency electrical signals it produces as a result of detecting the high frequency acoustic signal (note that other types of microphones also have dynamic range limitations, even if only due to mechanical limitations on the motion of their components). This means that the microphone's output will consist of electrical signals which would normally indicate the detection of both low and high frequency acoustic signals, and these electrical signals will then be sent to the filtering and signal processing stages of the alarm system. The result is that an intrusion alarm signal can be generated in the presence of only a high frequency acoustic signal, in which case the alarm signal is a false alarm.
What is desired is a means of reducing the probability of a false alarm being generated in response to the saturation of the sensor in an intrusion alarm system which uses a single microphone to detect various acoustic components of a glass break event.